In today's cellular network radio communication system (for example, LTE and WiMAX), user equipment (UE) receives not only a signal of a serving base station but also interference between neighboring base stations (i.e., inter-cell interference). Inter-cell interference becomes comparatively stronger and becomes a major factor to limit system throughput when the user is at the cell edge.
Multiple base station cooperation is a type of technology for effectively reducing interference between cells. Coordinated beamforming is a type of method that can realize multiple base station cooperation. When a plurality of antennas are placed at a base station, the antenna directivity can be changed according to a precoding vector of an antenna array, a signal of a serving cell can be increased, and at the same time, the interference from the neighboring cells can be reduced.
FIG. 1 is a model of a coordinated beamforming between each communication cell in a communication system. As shown in FIG. 1, the system includes three (but not limited to three) base stations (cells) eNB 1, eNB 2, and eNB 3 in which, base station eNB 1 is the serving base station of the user equipment (UE) located at the border of the three cells. UE receives a signal from its own serving base station eNB 1 but at the same time receives interference from the neighboring cells (base stations eNB 2 and eNB 3). User equipment UE needs to measure the serving cell channels and the neighboring cell channels and then give a periodic feedback to serving base station eNB 1 about channel information on these channels. As a result, serving base station eNB 1 designs a precoding vector of the beam forming and strengthens the signal of serving cell eNB 1, and notifies the channel information on the corresponding channel through an inter-base station communication to the base station of the corresponding cell; and reduces the interference to the serving cell eNB 1 by adjusting the precoding vector of its own self beam forming in these base stations.
The channel information herein may be PMI (Precoding Matrix Index) of each channel or CSI (Channel Status Information), and the like.
FIG. 2 is a schematic diagram illustrating a channel information report to base station eNB 1 of the serving cell of user equipment UE. As shown in FIG. 2, along with the passage of time, user equipment UE reports the all channel information of the three cells, such as precoding matrix index PMI-1 of serving cell eNB 1, and precoding matrix index PMI-2 and precoding matrix index PMI-3 of two neighboring cells eNB 2 and eNB 3, to serving base station eNB 1 of the serving cell once per cycle T.
In the above coordinated beamforming, user equipment UE needs to give not only a feedback on the channel information of serving cell eNB 1 but also a feedback on the channel information of the neighboring cells, and thus, as compared to the conventional system without the cooperative transmission between cells, the coordinated beamforming requires a larger feedback overhead. Therefore, a challenge of the coordinated beamforming is to effectively reduce feedback overhead.